1. Field of the Invention
The present invention relates to apparatus and method for measuring in a short period of time formaldehyde emissions from composite wood products bonded with urea-formaldehyde resin adhesives. The invention particularly relates to apparatus and method for monitoring formaldehyde emissions of small samples of a composite wood product that gives results equivalent to those obtained using the large chamber test method adopted in 24 C.F.R. 3280.208 as the industry standard for measuring formaldehyde emissions from composite wood products.
2. Description of Related Technology
Composite wood products made from urea-formaldehyde (UF) resin adhesives, such as particleboard or medium density fiberboard (MDF), find use in a variety of applications. In many of these applications, the wood products are used in areas of restricted or limited ventilation, such as in the construction of mobile homes. Since formaldehyde off-gassing or emission from UF resin bound composite wood products is a potential problem in such applications, standards, such as those promulgated by the United States Housing and Urban Development Agency at 24 C.F.R. 3280.208, have been established defining the permissible maximum level of formaldehyde emission from UF bonded composite wood products. Unless wood products made with a UF resin adhesive meet these emissions standards, they cannot be certified for use and sale in the regulated applications.
A variety of methods have been developed to measure formaldehyde emissions from the planar surfaces of such composite wood products. Basically, the formaldehyde testing methods (FTM) fall into two categories: full scale tests, which are designed to give results comparable to the environment encountered in actual use, and lab tests, ostensibly for quality control monitoring, designed to mimic the results obtained using the large scale test protocol. The first full scale method, the FTM-2 test, was developed by the Hardwood Plywood Manufacturers Association (HPMA) and the National Particleboard Association (NPA). This test has since been modified and adopted in 24 C.F.R. 3280.208 as the standard test for determining formaldehyde emissions from particleboard products such as underlayment, mobile home decking, industrial board and the like. See, "Large Scale Test Method for Determining Formaldehyde Emission from Wood Products - Large Chamber Method FTM-2 - 1985", National Particleboard Association et al., Nov. 11, 1985 which is herein incorporated by reference. Wood products which show excessive levels of formaldehyde emission under this test protocol cannot be certified for sale as part of the NPA and HPMA Grademark Program for particleboard and hardwood plywood.
The FTM-2 test uses a test chamber of at least 20 m.sup.3 (at least 800 ft.sup.3) into which a plurality of full-sized (48-60 inches in width) panels are placed spaced apart by about 6 inches. Air is moved through the chamber at a rate sufficient to assure good mass transfer (eddy diffusion mass transfer) from the entire surf aces of the panel samples. Make-up air also introduced into the chamber, at a rate relative to the test chamber volume, to achieve a constant gas hourly space velocity (GHSV) or rate of renewal (Q/V) as calculated by dividing the make-up air f low rate (Q) by the test chamber volume (V). Under the regulatory program different board products are tested at different product loading conditions.
The boards first are conditioned for 7 days at 75.degree. F. (about 24.degree. C.) and 50% relative humidity prior to testing. The samples then are placed into the test chamber and after 16-20 hours, the level of formaldehyde emission in an FTM-2 test system is presumed to have achieved steady state. The steady state level of formaldehyde emission is then determined by measuring the formaldehyde concentration (C.sub.S) in the circulating air. Formaldehyde concentration in the chamber air is measured in the FTM-2 test by bubbling a known volume of the chamber air through an impinger containing an aqueous solution of 1% sodium bisulfite. The bisulfite solution is then analyzed using the chromotropic acid method and a spectrophotometer to determine its formaldehyde concentration. Using known techniques, the measured value is converted into a formaldehyde emission value for the tested boards and compared against the established standards.
Although the FTM-2 test provides an accurate indication of the formaldehyde emission characteristics of the tested panels under the test conditions, the FTM-2 method is not suitable for routine process quality monitoring in a manufacturing facility. Not only is the long test interval prohibitive for measuring C.sub.S, including the week-long conditioning period and almost a full day for the test, but the FTM-2 test is also deficient as a method for assessing equilibrium formaldehyde emission (C.sub.eq) and the mass transfer coefficient (K) for the composite wood products.
A laboratory test purportedly developed to provide data that correlates to the full-sized FTM-2 test has been named the FTM-1 test. The FTM-1 test (also known as "the desiccator test") is a faster test method, which after a 24 hour conditioning period, requires only about 2 hours before a result is obtained. See, "Small Scale Test Method for Determining Formaldehyde Emissions from Wood Products, Two Hour Dessicator Test FTM-1 - 1983", National Particleboard Association et al., October 10, 1983 which is herein incorporated by reference.
To conduct the FTM-1 test, a plurality of small samples are placed in a desiccator containing a petri dish with 25 ml of distilled water. After 2 hours a sample of the water (about 4 ml) is analyzed by the chromotropic acid method wherein the concentration of formaldehyde is measured by monitoring the color of the solution using a spectrophotometer.
The FTM-1 test, however, has a number of shortcomings. First, the test procedure relies on molecular diffusion from the test specimens into the solution rather than on the more efficient eddy mass transfer obtained under conditions prevailing in the large scale test chamber. Secondly, the FTM-1 measuring method places an aqueous solution in the same environment as a formaldehyde-emitting surface. It is well known that formaldehyde emission from a composite wood product is strongly influenced by the conditions of exposure, particularly the prevailing humidity. The aqueous solution necessarily affects the humidity in the desiccator and thus the rate of formaldehyde emission from the tested samples.
Finally, the nature of the test is such that the same test results are obtained even when the total surface area of the wood composite samples is changed, indicating that the desiccator does not respond in a way similar to the large scale test chamber. This results in part because the amount of formaldehyde that is absorbed by the test solution in the FTM-1 test disturbs the emission equilibrium of the composite wood product sample and thus affects the amount of formaldehyde emitted from the samples contained in the test chamber. In other words, the aqueous solution acts as a formaldehyde sink increasing formaldehyde emission from the test samples rather than simply serving as an impartial monitor of equilibrium formaldehyde concentration in the test chamber. See Kinectics for Desiccator Jar and Alike Tests for Formaldehyde Release from Particleboard, Rybicky, Jaroslav, Wood & Fiber Science, Vol. 17, No. 1, January 1985.
Because of these and other inadequacies in the FTM-1 procedure, attempts to correlate its emission tests results with results obtained using the large scale chamber (FTM-2), which as noted above serves as the regulatory standard, have been less than satisfactory. Consequently, saddled with such testing inadequacies, mills producing composite wood products have been forced to operate with a wide production margin to ensure that certified board products meeting the formaldehyde emission limits of the appropriate standards are consistently obtained. Moreover, because only a single measure of formaldehyde can be determined, the FTM-1 test is deficient as a quality control tool in the manufacturing environment even if consistent results are obtained. As noted above, a test procedure that also determine the mass transfer characteristics of the composite wood product is needed if it is to be useful for quality control. Therefore, it would be desirable to have an alternative test to the large scale FTM-2 procedure that would measure formaldehyde emissions quickly (e.g., in less than about 30 to 60 minutes), conveniently and inexpensively with good correlation to FTM-2 test results.
One might think that it would be an easy matter merely to scale down the FTM-2 test chamber while continuing to use the corresponding make-up air flow rate (GHSV or Q/V) and loading ratio, i.e., the ratio of sample area (A) to chamber volume (V) used in the FTM-2 protocol. In other words, using a smaller chamber designed for operation at the same constant Q/V with the same corresponding A/V to imitate the large scale chamber. Unfortunately, if the air sampling and monitoring protocol remains the same as in the FTM-2 test, i.e., a sampling flow rate of 1 liter/min. to ensure identical results, the minimum chamber size for the regulatory required Q/V of 0.5 would be 0.12 m.sup.3 (about 4 ft.sup.3), a not inconsequential size. The device would be smaller than the large scale test chamber but not yet useful as a quality control monitoring method because steady state conditions would still require 16-20 hours to achieve. Moreover, the reduced absolute air flow and emitted formaldehyde volume would have to be measured by some method. Through the impinger technique of the FTM-2 protocol could be used to measure steady state (C.sub.S) formaldehyde emission from such a scaled down chamber, it can not be used to measure an equilibrium emission level because of formaldehyde mass balance and humidity problems also encountered with the FTM-1 method. Furthermore, the impinger technique for determining the steady state formaldehyde C.sub.S concentration itself requires a minimum of about 30 minutes to conduct to ensure an accurate determination of the formaldehyde emission level, and this is in addition to the time needed to reach steady state conditions.
It would be desirable to have a system for accurately measuring formaldehyde emission from composite wood products under both steady state and equilibrium conditions in less than about 30 to 60 minutes so that it could be correlated to the FTM-2 results and also serve as a useful quality control tool in the manufacturing environment.